I read somewhere online that Diane Duane’s Rihannsu books omnibus (Bloodwing Chronicles, I think it was called) was rewritten to fit in with Trek’s twisted, evil, imagination-bereft ‘canon’. Is this true?
No, it isn't. The books were written quite a few years apart, and Duane's own assumptions changed in the interim, so they contradicted each other on a few points. The slight revisions that were made were done merely in order to make them consistent with one another. There were also some textual revisions to conform to current editorial standards for capitalization, punctuation, etc.
Duane should have done her homework on these details to begin with, as failure to follow canon on such issues is sheer inexcusable incompetence and laziness.
I trust you're joking. You can't do homework on things that don't exist yet. When Duane's first few novels were written, Trek continuity was still not very well pinned down; there were different schools of thought on how much time elapsed between TOS and TMP. Duane's
The Wounded Sky, My Enemy, My Ally, and
The Romulan Way were written to take place in a hypothetical second 5-year mission before TMP -- an idea that would've put the interval between TOS & TMP closer to its real-life duration (although it does overlook the repeated TMP references to Kirk's "five years out there," though that's a problematical reference anyway). So the characters still had their TOS ranks, but the ship's design and crew composition were more evocative of TMP. Then
Spock's World was set post-TMP.
But quite some time later, when
Swordhunt/Honor Blade came out, a lot had changed in the Trek universe, and the idea of a second pre-movie 5-year tour had become unviable. The fix decided on by Duane and her editor (John Ordover, I believe) at the time was to retcon the books to take place before TMP, despite what
Spock's World had stated. However, it was another five years and a change of editor before
The Empty Chair came out, and by then Duane and her new editor had decided that it made more sense to put the whole series post-TMP, so the earlier Rihannsu books were tweaked to reflect that setting. Characters were given their TMP-era ranks, a couple of date references were modified, and a reference to Chapel studying for her doctorate was cut. (However,
The Wounded Sky and
Spock's World have not been so revised and still represent the original Duane interpretation of the chronology.)